The synthesis of mesoporous silicate solids using surfactant templating was discovered more than a decade ago. See, e.g., Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710, which is hereby incorporated by reference. A variation of the procedure, known as evaporation-induced self-assembly (EISA), involves confining all silica precursor and templating surfactant species within droplets. The solvent then progressively evaporates and that leads to an increase of the concentration of templating surfactant which, upon surpassing the critical micelle concentration, assembles into spherical or cylindrical micellar structures. See, e.g., Brinker, C. J.; Lu Y. F.; Sellinger, A.; Fan H. Y. Adv. Mater. 1999, 11, 579-585, hereby incorporated by reference. When the solvent is fully evaporated, the silica solidifies around the surfactant structures. This is followed by surfactant removal via calcination, resulting in the formation of a well ordered mesoporous silica material. EISA has been successfully utilized to fabricate well-ordered thin silica films and particles using a wide range of surfactants and block copolymers. See e.g., Brinker, C. J.; Lu Y. F.; Sellinger, A.; Fan H. Y. Adv. Mater. 1999, 11, 579-585 and Lu, Y.; Fan, H.; Stump, A.; Ward, T. L.; Riker, T.; Brinker, C. J. Nature (London) 1999, 398, 223-226, each of which is hereby incorporated by reference. However, the mesoporous silica particles obtained by EISA are usually characterized by substantial polydispersity.
Recently, Andersson et al. (Andersson, N.; Kronberg, B.; Corkery, R.; Alberius P. Langmuir 2007, 23, 1459-1464, incorporated by reference), demonstrated the synthesis of spherical, mesoporous silica particles using an approach which combines previously established emulsion-based precipitation methods (See, e.g., Schacht, S.; Huo, Q.; Voigt-Martin, I. G.; Stucky, G. D.; Schuth, F. Science (Washington, D.C.) 1996, 273 (5276), 768-771 and Huo, Q.; Feng, J.; Schueth, F.; Stucky, G. D. Chem. Mater. 1997, 9, 14-17, both incorporated by reference) with the EISA method. This synthesis route, referred to as the emulsion and solvent evaporation method (ESE), produced well-ordered 2D hexagonal mesoporous silica microspheres. The emulsions were prepared in bulk using inhomogeneous vigorous stirring. As a result, the droplets, and therefore the particles, were produced with a relatively broad size distribution. FIG. 1 shows a histogram and FIG. 2 a Scanning Electron Microscopy (SEM) image of particles obtained from a polydisperse bulk emulsion. As easily seen in the figures, the size distribution is broad and includes a wide range of particles.
The fabrication of monodisperse silica microparticles containing highly ordered nanometer-scale pores (mesopores) of controllable size presents a fundamental challenge and is of practical interest. See e.g., Rama Rao, G. V.; Lopez, G. P.; Bravo, J.; Pham, H.; Datye, A. K.; Xu, H.; Ward, T. L. Adv. Mater. 2002, 14, 1301-1304, which is hereby incorporated by reference. The microparticles can be used in a variety of applications including, but not limited to, controlled drug delivery, molecular, biomolecular and cellular encapsulation. See, e.g., Lou T.-J. M.; Soong R.; Lan E.; Dunn B.; Montemagno B. Nature Materials 2005, 4, 220-224 and Chia, S. Y.; Urano, J.; Tamanoi, F.; Dunn, B.; Zink, J. I. J. Am. Chem. Soc. 2000, 122, 6488-6489, both of which are hereby incorporated by reference. For example, monodisperse particles can be ordered into 2D and 3D arrays or lattices which allow the fabrication of catalysts with well-defined pore hierarchy. See e.g., Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Langmuir 1992, 8, 3183-3190 and Dimitrov, A. S.; Nagayama K. Langmuir 1996, 12, 1303-1311, both of which are hereby incorporated by reference. Mesoporous particles also have significant potential for the design and implementation of chemical and biochemical sensors, as described, for example, in Buranda, T.; Huang, J.; Ramarao, G. V.; Ista, L. K.; Larson, R. S.; Ward, T. L.; Sklar, L. A.; Lopez, G. P. Langmuir 2003, 19, 1654-1663, which is hereby incorporated by reference.
The present disclosure provides novel methods and devices for forming monodisperse populations of microparticles. In some embodiments, the microparticles may be formed into 2- or even 3-dimensional arrays useful for a variety of applications.